PU coated garments, particularly gloves, are commonly produced by the method of dip coating. This involves the dissolving of PU resins with DMFa, along with other components such as processing aids and colorants, to provide viscous flowing dipping compounds specifically for glove coatings. The coatings in question are often coloured black when the gloves are intended for industrial purposes and the black pigment used is typically carbon black.
It is well known in the glove business that PU gloves contain residual DMFa from the processing of the gloves. There have been many publications on the matter of the use of DMFa in PU glove production. The patents/published applications U.S. Pat. No. 8,894,896, EP 192193761, U.S. Pat. No. 8,241,705 and WO 2008029703 A1 both discuss the processing of PU resin based gloves with polar aprotic solvents, such as DMFa, and describe the dangers of residual DMFa in PU gloves along with trying to provide alternative materials and processes.
It is known that DMFa is a dangerous chemical and exposure to high levels should be avoided. It is a known allergen and can be absorbed through the skin. Excess exposure can cause liver malfunction and other health complications. Hence every effort is made to remove the residual DMFa from the gloves during processing.
Currently, testing for the level of residual DMFa in work gloves is being introduced in the new European Standard EN 420 for safety gloves and where a specified extraction method of EN 16778 is established to determine reasonably accurately the amount of the residual DMFa contaminant.
Surprisingly, what is less known is the fact that there is generally far more residual DMFa left over in black coloured compound dipped gloves as compared to both grey coloured gloves (where a small amount of carbon black is used in combination with a white pigment, usually titanium dioxide, in their colouration) and white gloves. The pigment base being used across the market to make black PU gloves is carbon black. It is the pigment of choice due to both practicality and cost. In fact, carbon black is used to colour most polymer bases for articles.
Most gloves being made for use in industry are black or grey, and they both contain carbon black pigment. The amounts of black pigment used with titanium dioxide to create grey colour are very small, typically from 0.01% to 1 or 2% by weight, depending on the darkness of grey required. Those practised in the art will know of the blends used to create the many shades of grey. These tiny amounts of carbon black still have effects on anchoring and holding residual aprotic solvents in the gloves, as explained below.
The problem is that carbon black has both a unique chemistry and physical nature. The microscopic particles of carbon black are highly structured materials that tend to have huge relative surface areas. The chemical nature of carbon black means that it has a tendency to attract and lock on to organic solvents and organic materials by adsorption, holding on to them within the black coloured polymer matrix. This property of chemical attraction is referred to as the ‘active effect’ in the following description.
As mentioned above, the pigment used in white coloured gloves is titanium dioxide. Unlike carbon black, titanium dioxide does not offer any significant chemical attraction ‘active effect’ to DMFa or other organic solvents used during the manufacturing process. Consequently, residual DMFa is significantly less of a problem with white gloves.
Activated charcoal (further modified carbon black) is used in applications where the adsorption and holding of hazardous organic materials and solvents is an essential requirement. Such uses include in respirator cartridges and also nuclear, biological and chemical (NBC) warfare suits—where, for example, mustard gas protection may be required. In the case of respirator cartridges, they are packed with grains of activated charcoal and as one breathes fumes along with air through the cartridges the solvents are ‘mopped up’ out of the air. Similarly, with NBC suits there is a layer of activated charcoal impregnated foam running in the middle of laminated fabric as a sandwich to adsorb organic vapours and so prevent the transfer of the hazardous organic chemical to the wearer.
In the case of black PU gloves, it is this property of the carbon black that is very undesirable as the carbon holds onto the residual DMFa that one would want to remove during the manufacturing process. The carbon restricts the DMFa from being leached out of the coating during the process. Nor does it fully allow the evaporation of the DMFa from the coating at higher temperatures of glove cure in the final ovens. Such are the problems of black gloves that many manufacturers and sellers introduce additional washing and drying programs to reduce the relatively high DMFa levels.
Another problem with carbon black is residual poly aromatic hydrocarbons (PAH). They are another hazardous listed and restricted chemical group—according to DIN EN 1186ff and s 64 LFGB 80.30 and GS symbol concession according to AfPS GS 2014:01 PAH. Though some carbon black pigments available on the market have been ‘double processed/refined’ to remove residual PAHs—these pigments are in limited supply and are expensive.
Hence there is a need to reduce or remove carbon black pigments from PU fabric and glove dipping formulations. Surprisingly, this is not so easy. There are alternative black pigments available in the pigment industry. The ones generally being used are metal oxide based, such as iron oxide. However, these pigments are very weak and do not have the strength of shade that carbon black has. The strength and depth of the black is known as the “jetness”; hence the term “jet black”. There are also organic black pigments that are both expensive and also not strong enough.
Formulations made with the direct replacement of carbon black by iron oxide give a blue grey coloured dip glove coating—not at all jet black. Even with double the loading of iron oxide, it simply does not have the effective strength of blackness required.
The U.S. Pat. Nos. 7,122,593 and 7,635,733 describe latex gloves which contain dispersed chromium oxide particles and are thereby rendered magnetically detectable. The published patent application US 2004/0154072A1 relates to gloves that contain electromagnetically detectable particles, such as iron oxides or other ferrous materials. All of these gloves are intended to be worn by persons working in areas such as the preparation of food products, so that fragments of the gloves that may become detached or torn off during use can be easily detected and do not contaminate the food products.